Abstract: A synchronizing circuit receives an external signal and yields an output that is synchronized with the system clock and operates at the frequency of the external signal. The signal output from the synchronizing circuit is fed into the clock-enable input of the storage element, and the system clock signal is fed into the clock input of the storage element. Because the clock-enable signal triggers the storage element, the storage element is driven at the external signal frequency. Clock skew is eliminated because the system clock used for the clock input to the storage element is skew-controlled.

Abstract: A separate cacheable-in-virtual-cache attribute bit (CV) is maintained for each page of memory in the translation table maintained by the operating system. The CV bit indicates whether the memory addresses on the page to which the translation table entry refers are cacheable in virtually indexed caches. According to a first embodiment, when there are two or more aliases which are not offset by multiples of the virtual cache size, all of the aliases are made non-cacheable in virtually indexed caches by deasserting the CV bits for all aliases. With regards to the contents of the translation lookaside buffer (TLB), the translations for all aliases may simultaneously coexist in the TLB because no software intervention is required to insure data coherency between the aliases. According to second and third embodiments of the present invention, when there are two or more aliases which are not offset by multiples of the virtual cache size, only one of those aliases may remain cacheable in virtual caches.

Abstract: An apparatus for processing a write miss signal from a copy-back data cache includes a load-store unit with an allocating load buffer, a non-allocating store buffer, and a priority control circuit to generate write-after-read hazards and read-after-write hazards to preserve the processing priority of entries within the allocating load buffer and the non-allocating store buffer. A prefetch circuit enqueues a prefetch command in the allocating load buffer and a store command in the non-allocating store buffer upon a write miss to the copy-back data cache. Thus, the priority control circuit forces a write-after-read hazard on the store command in the non-allocating store buffer. As a result, the prefetch command in the allocating load buffer secures an allocated line in the copy-back data cache, allowing the store command of the non-allocating store buffer to write data to the allocated line.

Abstract: A method and apparatus for dynamically switching the relative priorities of the load buffer and store buffer with respect to external memory resources in a superscalar processor. According to a first embodiment, a protocol dictates that the load buffer always prevails until the store buffer reaches a certain "high water mark," (an upper threshold) at which time the store buffer gains priority. After the store buffer has gained priority, it continues to access the memory until it is depleted to a "low water mark," (a lower threshold) at which time the load buffer regains priority. Whenever the store buffer reaches the high water mark, it gains priority until it drains down to the low water mark. This reduces the tendency for the store buffer to become full and block the processor. According to a second embodiment, the load buffer prevails if it is above its high water mark.

Abstract: A dual-ported tag array of a cache allows simultaneous access of the tag array by miss data of older LOAD instructions being returned during the same cycle that a new LOAD instruction is accessing the tag array to check for a cache hit. Because a load buffer queues LOAD instructions, the cache tags for older LOAD instructions which missed the cache return later when new LOAD instructions are accessing a tag array to check for cache hits. A method and apparatus for calculating and maintaining a hit bit in a load buffer perform the determination of whether or not a newly dispatched LOAD will hit the cache after it has been queued into the load buffer and waited for all older LOADs to be processed. A load buffer data entry includes the hit bit and all information necessary to process the LOAD instruction and calculate the hit bits for future LOAD instructions which must be buffered.

Abstract: A dual-ported tag array of a cache allows simultaneous access of the tag array by miss data of older LOAD instructions being returned during the same cycle that a new LOAD instruction is accessing the tag array to check for a cache hit. Because a load buffer queues LOAD instructions, the cache tags for older LOAD instructions which missed the cache return later when new LOAD instructions are accessing a tag array to check for cache hits. A method and apparatus for calculating and maintaining a hit bit in a load buffer perform the determination of whether or not a newly dispatched LOAD will hit the cache after it has been queued into the load buffer and waited for all older LOADs to be processed. A load buffer data entry includes the hit bit and all information necessary to process the LOAD instruction and calculate the hit bits for future LOAD instructions which must be buffered.